1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly, to a liquid crystal display device using a ferroelectric liquid crystal.
2. Description of the Related Art
A conventional liquid crystal display device using a nematic liquid crystal comprises a twisted nematic (TN) type liquid crystal display device and a supertwisted birefringence effect (SBE) type liquid crystal display device.
In the twisted nematic type liquid crystal display device, however, as a driving method becomes multiplex, a driving margin becomes narrow, so that sufficient contrast cannot be obtained. In the supertwisted birefringence effect type liquid crystal display using a large twist angle, which is an improved display device of the twisted nematic type liquid crystal display device, when it is used in a large-capacity display device, the contrast is lowered or the response speed is decreased.
Then, in order to improve the liquid crystal display device using the nematic liquid crystal, there was proposed a liquid crystal display device using a chiral smectic C liquid crystal, that is, a ferroelectric liquid crystal by N. A. Clark and Lagerwall in 1980 (Japanese Unexamined Patent No. 107216/1981, and U.S. Pat. No. 4,367,924).
This liquid crystal display device is different from the above liquid crystal device using an electric field effect in which the dielectric anisotropy of a liquid crystal molecule is used and has a structure employing a rotating force for matching a polarity of a spontaneous polarization of the ferroelectric liquid crystal with a polarity of the electric field. This liquid crystal device has a bistable characteristic, a memory characteristic, and high-speed response characteristic. More specifically, when the ferroelectric liquid crystal is injected into a cell having a thin gap, a spiral structure of the ferroelectric liquid crystal is broken by an influence of an interface of the substrate, and then there exist together a region where the liquid crystal molecule is inclined from the normal line of the smectic layer by an angle of .THETA. and a region where the liquid crystal molecule is inclined in the opposite direction by -.THETA., so that a bistable property is provided. The directions of the liquid crystal molecule and its spontaneous polarization can be uniformby applying a voltage to the ferroelectric liquid crystal in the cell. Therefore, by switching the polarity of the voltage to be applied, the orientation of the liquid crystal molecule can be switched from a certain constant state to another constant state.
Since birefringent light in the ferroelectric liquid crystal in the cell is changed by the switching operation, transmitted light can be controlled by sandwiching the cell between two polarizers. In addition, even if the application of the voltage is stopped, the orientation of the liquid crystal molecule is kept at a state before application of the voltage is stopped, so that the memory effect can be also obtained. A time required for driving the switching operation has a high-speed response characteristic, that is, below 1/1000 of the twisted menatic type liquid crystal display device because the spontaneous polarization of the liquid crystal and the electric field directly affect it, enabling high-speed display. Thus, by using the memory effect or the high-speed response characteristic of the ferroelectric liquid crystal, there has been implemented a liquid crystal display device of high resolution in which there are many scanning lines using a multiplex driving method.
However, there are many problems in the liquid crystal display device proposed by Clark and Lagerwall. That is, it is thought that in a first model, a layer structure of the smectic C phase has a structure perpendicular to a substrate, called bookshelf type shown in FIG. 3.
However, in a case where a cell is made by using a conventional orientation method by rubbing or the like, it has been found that a switching phenomenon and an optical characteristic is considerably different from that expected, and its switching operation is completely different from that of the proposed model.
As one reason for the above, it was analyzed using a small angle scattering method of an X-ray that the layer structure is doglegged, called a chevron structure shown in FIG. 4. [Phys. Rev. Lett., 59, pp.2658 (1987) by Rieker, T. P., Clark, N. A,]
In addition, it is also different from the first model in that the direction of the spontaneous polarization and the liquid crystal molecule have a uniform orientation and also the molecule has a twisted orientation between upper and lower substrates. [Glogarova, M. and Pavel, J., J. Phys. (France), 45, pp. 143 (1984)]
Especially, the ferroelectric liquid crystal display device oriented by rubbing has the twisted orientation because regulating force at the interface strongly acts thereon. In such orientation, it is found that a difference in an optical molecular axis in switching between two states does not generally effectively appear and high contrast characteristic is not obtained. In order to solve the above problems, there are proposed several methods to attain a layer structure model proposed by Clark et al. For example, there is one example in which an SiO oblique deposition method is used, and then the layer is prevented from being doglegged by applying a relatively high pretilt to the substrate interface to attain an oblique layer structure.
In addition, there is another method in which the layer structure is changed to the bookshelf structure by applying an alternating electric field of a high voltage to the cell having the doglegged structure [the twelfth Liquid Crystal Meeting (Nagoya) by Sato et al., 1F16 (1986)]. It has been reported that high contrast can be provided by either method.
However, in the above oblique deposition method, it is difficult to make the deposition angle uniform and there is a big problem in productivity because of vacuum processing. In addition, in the method which applies the electric field, it is difficult to uniformly change the layer structure because it gradually changes to the chevron structure with the lapse of time, so that it has not been put to practical use.